Tracking the spread of culture through folktales

There’s a reason why the premise of American Gods is so alluring: the US is home to a wild and glorious mishmash of gods, folktales, and cultural heritage. One by one, groups from around the world picked up and landed on a new shore, bringing their stories with them.

The mere existence of certain tales can be revealing. They develop and mutate as they get passed from one group to the next, and the best stories are passed on more readily. Understanding the spread of folktales can help us understand cultural evolution more generally, and a paper in this week’s PNAS does just that by combining data on folktales with genetic, geographic, and linguistic information.

Researchers studying cultural evolution use biological evolution as a starting point for their ideas, but they also point out that clear and important differences separate the two types of evolution. The timescales, for instance, are often very different—cultural units can be transmitted between people of the same generation, while powerful ideas (like religions) can spread incredibly quickly and easily. A lot of work in cultural evolution is dedicated to trying to divine the mechanisms that underlie the spread of cultural ideas. How do people choose which ideas to adopt? And how does the spread of ideas compare to the spread of genes?

Migration vs. diffusion

There are different ways for folktales to spread. One is the American Gods-style model: people move and take their stories with them. But stories can also spread without migration, as they get passed among neighboring populations through “cultural diffusion.” For instance, you might be more likely to know what the deal is with St. Patrick and the snakes if you grew up around Boston rather than Albuquerque.

That’s where the genomic data comes in. If stories are diffusing through neighboring populations, you’ll see a similar pattern across genes, stories, and geography: places that are closer together will have…